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AS · Practice questions · Relative masses

Weigh it against carbon.

Six original Cambridge-style questions on the carbon-12 scale: definitions, Ar from isotopic abundances, abundances from Ar, Mr and formula mass, and the mass spectrum of a diatomic molecule.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Everything is compared with carbon-12.

01
Recall
[2 marks]

Define relative atomic mass.

  • The weighted mean mass of an atom of an element, ✓
  • relative to 1/12 of the mass of an atom of carbon-12. ✓
02
Calculation
[2 marks]

A sample of magnesium contains 78.99% 24Mg, 10.00% 25Mg and 11.01% 26Mg. Calculate the relative atomic mass of magnesium to three significant figures.

  • Sum of mass × abundance: 24 × 78.99 + 25 × 10.00 + 26 × 11.01 = 1895.76 + 250.00 + 286.26 = 2432.02. ✓
  • Ar = 2432.02 ÷ 100 = 24.3. ✓
03
Calculation
[2 marks]

Gallium has two isotopes, 69Ga and 71Ga. Its relative atomic mass is 69.72. Calculate the percentage abundance of each isotope.

  • Let x be the percentage of 69Ga: 69x + 71(100 − x) = 6972. ✓
  • 7100 − 2x = 6972, so x = 64.0. 69Ga is 64.0% and 71Ga is 36.0%. ✓
04
Calculation
[2 marks]

Calculate the relative molecular mass of ethanol, C2H5OH, and the relative formula mass of ammonium sulfate, (NH4)2SO4. (Ar: H 1.0, C 12.0, N 14.0, O 16.0, S 32.1)

  • Ethanol: 2 × 12.0 + 6 × 1.0 + 16.0 = 46.0. ✓
  • Ammonium sulfate: 2 × (14.0 + 4.0) + 32.1 + 4 × 16.0 = 36.0 + 32.1 + 64.0 = 132.1. ✓
05
Analysis
[3 marks]

Natural chlorine is 75% 35Cl and 25% 37Cl. Predict the m/z values and the relative heights of the molecular ion peaks in the mass spectrum of Cl2. Show your reasoning.

  • Possible molecules: 35Cl35Cl (m/z 70), 35Cl37Cl (m/z 72) and 37Cl37Cl (m/z 74). ✓
  • Probabilities: 0.75 × 0.75 = 0.5625; 2 × 0.75 × 0.25 = 0.375 (the mixed molecule forms two ways); 0.25 × 0.25 = 0.0625. ✓
  • Relative heights 70 : 72 : 74 = 9 : 6 : 1. ✓
06
Explain
[2 marks]

Explain why the term relative formula mass, and not relative molecular mass, is used for sodium chloride.

  • Sodium chloride is a giant ionic lattice of Na+ and Cl− ions; it contains no discrete molecules. ✓
  • The mass is therefore quoted for one formula unit, NaCl, the simplest ratio of ions in the lattice. ✓

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